A. Field of the Invention
The invention relates to an efficient method for the redistribution of radiant, particularly electromagnetic, energy for illumination purposes. More particularly, it is a practical method for the efficient distribution of pseudo-collimated light energy into a pre-determined image pattern with controlled intensity in the pattern.
B. Problems in the Art
Numerous situations require or benefit from the controlled distribution of radiant energy. Many times the source of radiant energy is omni- or non-directional. Many advantageous functions are possible by controlling its direction, distribution, and intensity.
For the purposes of this disclosure, the terms light, radiant energy and electromagnetic energy may include portions of the electromagnetic spectrum in the visible region and may also include portions of the electromagnetic spectrum which lie outside the visible range (e.g. infrared and ultraviolet).
There are two branches of optical design with regard to the distribution of electromagnetic radiation—imaging and non-imaging. Imaging optics can be defined as the science of transferring electromagnetic energy from an object plane to an image plane with minimal distortion. For the purposes of discussion, one can define the “object plane” or “input image” as a predefined energy input distribution, while the “image plane”, “image” or “output image” can be defined as a predefined energy output distribution which may vary in both intensity and direction. Typically, imaging optical systems significantly attenuate the input image energy in the process of minimizing the output image's distortion. The degree to which the imaging optical system attenuates the energy transfer depends upon the application. The energy attenuation of imaging optical systems is acceptable for applications such as cameras, microscopes, and the like. However, when the application is an illumination system, the primary goal is to maximize the energy throughput of the optical system. Hence, the other branch of optical design, non-imaging optics, is the science of maximizing the transfer of electromagnetic energy from a source image to an output image. Thus, non-imaging optics are particularly useful for illumination applications.
There are many situations where electromagnetic energy is required to be distributed into a pre-determined output image with a maximum transfer of source energy. For example, overland vehicle safety lighting, aircraft lighting, street lamp lighting, and marine lighting require specific output patterns, many times determined by government regulations which can have minimum and maximum illumination values and which vary substantially in different directions. In each such case, regulations typically specify minimal photometric requirements that must be met by the illumination device. In many of the above cases the required output distribution is rectangular in angle space and these applications are one aspect of what the present invention addresses.
It is difficult to achieve required output distribution efficiently and economically with imaging systems. Historically, in non-imaging solutions, designers used simplified surface geometry to approximate output images. However, this tends to result in non-optimal solutions which necessitate greater source energy and higher system power requirements. Output surfaces in typical collimated light solutions are comprised of a few simple shapes including ellipses, parabolas, radii, torroidal sections, and multi-radii surfaces. However, these surfaces limit the available distributions to meet a specific requirement. The result is that some areas of the pattern receive too much energy in order to meet required specifications, requiring a greater source energy and greater power consumption to meet requirements. Thus, it is also difficult to meet many required output distributions with non-imaging systems with efficacy and efficiency. With simplified optics such as used in conventional non-imaging illumination systems, it is also difficult to get precise control of light, particularly to distributed points throughout a required pattern.
An example illustrates the problems and deficiencies in the art. With respect to reverse or back-up illumination lights for semi-tractor trailers, regulations tend to require a minimum amount of light intensity within a rectangular pattern, measured within the light output of the reverse light. Not only are there minimums for light levels generally, but also minimums for specific regions or points throughout the rectangular pattern. In other words, it is not enough that a certain light level is met somewhere in the rectangle. There must be certain minimum light levels achieved at a variety of points or areas within the rectangle.
It is conventional to have a standard incandescent light bulb as the light source for reverse lights. It emits substantially omni-directional or a spherical ball light. Conventional semi-tractor trailer back up lights have a simple, circular shape. A housing and a simple, single, circular lens typically are the components that direct the light. This throws out essentially a big, rough circular beam or pattern of light. The intensity distribution of such a circular output pattern lamp can only be controlled relative to one axis and it does not match up well with a rectangular pattern requirement.
The most common technique used by optical designers to attempt to meet rectangular pattern requirements with a circular output pattern is by projecting a large circular pattern which covers the rectangle. However, this sends substantial light outside the rectangular pattern. This is inefficient. Light is wasted. For example, the number of lumens of light energy in a large circular pattern that subsumes the required rectangular pattern would be significantly higher than an efficient rectangular distribution that is basically limited to the required rectangular pattern. To ensure sufficient light intensity requirements are met throughout the rectangular pattern, the conventional inclination is to use higher intensity or larger light output light sources. This adds cost and adds to energy consumption. It can actually result in the inefficient generation of substantially too much light at all or most of the points in the pattern, just to ensure minimum requirements at all the points are met.
Thus, there is room for improvement in the art regarding a more efficient way to meet light requirements from a circular reverse light generating a circular light pattern to meet rectangular lighting requirements. In the case of semi-tractor trailer reverse lights, it is critical for safety that the driver have sufficient practical illumination of what is behind the trailer when backing. What is “sufficient practical illumination” balances amount of light with cost of light. High intensity flood lights could provide more than enough illumination. But, of course, energy usage and limited life of such lights are disadvantages. And an over-the-road vehicle does not have the luxury of unlimited electrical power. Size and placement on a tractor trailer also are limitations. Large bulky flood lights would be impractical. Robustness is also important. Fragile flood lights would not be practical. Also, there are practical limitations on how much light, and it what pattern, back lights can produce. Glare from high intensity flood lights would be impractical and unsafe on an over-the-road vehicle. Thus, an improvement must take into consideration economics and must overcome limitations regarding size, weight, and characteristics of the light.
Another design technique used in the past is to use a lens to modify the output pattern of the light. Such designs create a lens surface profile in one direction and sweep it through a profile in the second direction forming what is known in the art as a “pillow” lens. The result of this sweeping is that both the horizontal and vertical distribution can be controlled. However, these pillow-type lenses can only diffuse light to approximately 30 degrees. Examples of angular requirements for automotive lighting include the following: (a) stop, tail and turn lamps are generally −80 to +80 degrees horizontal, (b) reverse lamps are generally −45 to +45 degrees horizontal and (c) clearance, side marker and identification lamps are generally −45 to +45 degrees horizontal. As can be seen, these requirements substantially exceed the 30 degree capability of typical pillow type lenses. Therefore, they are not practical for many automotive illumination or lighting applications.
Historically, the preferred energy source for these lights was an incandescent lamp. Recent advances in technology have allowed light emitting diodes (LEDs) to be employed as the source energy. LEDs have many advantages over incandescent sources including longer life, faster turn on times, and low energy consumption. The main disadvantage of LED sources is the cost, which can be ten times that of an incandescent lamp.
It was due to the low cost of incandescent lamps that the practice of using simplified surface geometry for controlling light from these devices has been acceptable for decades. With the advent of LEDs, and the cost associated with these improved sources, the use of simplified optical geometries can be cost prohibitive. Using more complex optics with incandescent sources, or with LEDs, adds further to the cost. There is a real need in the art for a more cost-effective reverse light of this type.
The foregoing example of tractor-trailer reverse lights is illustrative of similar issues with other automotive lights. And similar issues exist with respect to other illumination applications, and other types of radiant energy, and for different output patterns or light distribution within required output patterns. The control of radiant energy from a source into a different output pattern in an efficient, effective, economical way is a need for many applications. This includes more efficient use of electrical power in their operation. Therefore, there is a need in the art for improvement regarding these types of issues and illumination applications.